A widely used predictively-coded video compression standard is that adopted by the Moving Picture Experts Group (MPEG). The MPEG-2 standard was initially proposed and developed as an open standard by the international standards organization (ISO), its main strengths are its flexibility and compression efficiency. MPEG achieves a high compression rate by using encoding techniques that remove much of the redundancy in the video signal. Spatial redundancy is reduced by applying a frequency-domain transform to blocks of picture elements (pixels) in the image and encoding the frequency-domain coefficients. Temporal redundancy is reduced by intra-frame encoding only a relatively few images (i.e. using only data in the image frame) while encoding most of the frames predictively, that is to say, based only on the changes from a previously encoded frame. MPEG encoded video is organized as groups of pictures (GOPs) including at least one intra-frame encoded image (I-frame) and which may include one or more predictively encoded images (P-frames) and one or more bidirectionally predictively encoded images (B-frames). Note that P-frames and B-frames are optional and need not be included in a GOP. Because of its high compression efficiency and because it accommodates a large variety of video formats, the MPEG-2 standard is well suited for data transmission and storage.
MPEG-2 video compression is lossy, that is to say, significant information is lost when the signal is compressed into I-frames, P-frames and B-frames. The standard is designed, however, to take advantage of the human visual system in order to hide these losses. Much of the lost information appears as quantization distortion of high spatial frequency components of the image. The human visual system is relatively insensitive to this type of distortion.
Because of the multiple encoding steps used to compress the video images, considerable processing is typically used to reproduce the image. The predictive decoding used to reproduce the image is problematic for trick-play features such as playing the video program in reverse. To reduce the complexity of such modes, many existing systems decode and display only the I frames of the GOP's for reverse play. Because these frames may be separated by, for example, 10 or more frame intervals, the human visual system detects the shortcomings of the strobe-like effect of successive images flashed on the screen at a frame rate of, for example, 6 Hz.
Accordingly, there is a need for a reverse decoding and display method offering a significant improvement in reverse play quality for predictively coded compression signals, such as MPEG-2 signals.